1. Field of the Invention
The present invention relates to an pressure-sensitive adhesive or adhesive (hereinafter simply referred to as “adhesive”) film and, more particularly, to an adhesive film which is excellent in cuttability and exhibits good behavior in such shearing process as punching or perforating.
The “film” so referred to herein includes, within the meaning thereof, tape-like, sheet-like and film-like forms.
2. Description of the Related Art
In various fields such as electric and electronic devices, medical devices/appliances, livingware, buildings and cars, adhesive films have so far been used for various purposes, for example in fixing or bonding parts or members, in manufacturing flexible printed circuit boards (hereinafter referred to as “FPCs”) obtainable by bonding together a copper foil and an insulating film such as a polyimide film or tapes for TAB (Tape Automated Bonding), mutually laminating rigid type printed circuit boards, fixing device elements, bonding reflective sheet or reflector aluminum/SUS sheets for liquid crystal displays, fixing casings and members via protective insulating layers, protecting cellular phone glass screens, fixing housing members and fixing car interior finishing materials.
Such adhesive films in current use include the so-called “baseless” adhesive films comprising an adhesive layer alone, and adhesive films having a base layer or sheet made of polyethylene terephthalate (PET) or a nonwoven fabric, for instance, with an adhesive-based surface layer or layers formed on one or both sides of the base (the term “pressure-sensitive adhesive” as used herein refers, among various adhesives, to an adhesive which has viscoelasticity in a solventless condition at ordinary temperature and, upon application of a pressure thereto, flows and provides a required level of bond strength; hereinafter the same shall apply).
The following document may be mentioned as related arts of the present invention:    Patent Document 1: Japanese Unexamined Patent Application Publication No. H9-208907 (Paragraphs [0002], [0003], etc)
According to the respective intended uses thereof, such adhesive films are cut to a predetermined size and form by punching or the like and, further, may be subjected to various processing such as perforating or slitting.
However, the conventional adhesive film comprising a base layer and a surface layer comprising adhesive, laminated on the base layer have various problems since the base layer and the surface layer greatly differ in physical properties.
For example, a double-faced adhesive film comprising, as the base layer, a biaxially stretched PET film excellent in dimensional stability is provided. In this constitution, however, the difference or ratio in shearing strength between the base layer and each surface layer is about 1:180 when the base layer has a thickness of 4.5 μm and the surface layer has a thickness of 25 μm; when the base layer has a thickness of 25 μm, that ratio is about 1:400.
Further, as to the elongation, a 4.5-μm-thick biaxially stretched PET film used as the base layer shows an elongation of 140%, and a 25-μm-thick surface layer shows an elongation of at least 400%; thus, the ratio between them is 1:3 or higher (cf. “Table 1”).
TABLE 1Shearing strengths and elongations of the conventional base layer(biaxially stretched PET film) and surface layer (acrylic adhesive)Base layerSurface layer(biaxially stretched PET film)(acrylic adhesive alone)ShearingShearingThicknessstrengthElongationThicknessstrengthElongation(μm)(g/25 mm)(%)(μm)(g/25 mm)(%)4.51800140510 or less400 or more1223001411510 or less350 or more2540001422510 or less350 or more
Thus, the base layer, which is low in elongation percentage and is rather hard and superior in rupturability, is better processible by punching, perforating or like cutting process, whereas the adhesive constituting the surface layer is flexible and very stretchable by nature; therefore, when the adhesive film is cut, the surface layer alone is drawn then peeled off from the base and, as a result, a burr (adhesive burr) is formed by the adhesive in the surface layer, possibly causing a problem from the product quality viewpoint.
In particular, in the case of adhesive films used in products required to have high levels of precision, such as in bonding electronic elements in cellular phones or digital cameras, and subjected to fine cutting, a parting thereof may be bent, adhesive burrs may transfer to installed parts/devices, or electronic devices may rise up on adhesive burrs, and product troubles such as failure to perform the desired functions may possibly be caused thereby.
Further, the above-mentioned adhesive burrs formed by shearing of the surface layer stick to working tools such as cutting dice, drills or slitters of shearing devices, causing troubles in the subsequent steps on the occasion of continuous shear processing, possibly leading to interruption of the production line. In the case of punching or press processing, for instance, high-precision inspections for transfer of adhesive burrs to processed products and for successful cutting out of adhesive films are required and, further, processes to remove adhesive burrs adhering to cutters and other working tools are required about every 40,000 shots, whereby the production process becomes complicated.
On the other hand, in the case of the conventional baseless adhesive films, the adhesive is sticky and stretchable by nature and, therefore, the formation of such adhesive burrs as mentioned above is unavoidable and, in addition, the adhesive burrs formed themselves have the function of adhesives, so that they readily get larger and cause further troubles. To overcome these problems, such measures are known as selection of increasing the molecular weight of the adhesive at a level of 1,000,000 to 8,000,000 or more and employment of a highly crosslinked formulation; however, these measures may readily cause decreases in tackiness and the adhesive properties at ordinary temperature may be sacrificed, and the glass transition point (Tg) is also increased and the adhesive properties at low temperature may be readily sacrificed.
The reason why the conventional adhesive films constituted of a biaxially stretched PET film base layer and an adhesive layer or layers thereon allow the formation of adhesive burrs upon such shearing process as punching or perforating is the occurrence of an interface between the base layer and the surface layer-constituting adhesive which differ in elongation percentage as a result of different compositions thereof, among others. In many cases, adhesive burrs are presumably formed as a result of peeling off of the surface layer comprising adhesive from the base as caused by failure of that interface.
Based on these premises, the inventors paid attention to the fact that the formation of adhesive burrs in the steps of slit formation and cutting is caused by the phenomenon of adhesion of low-molecular-weight uncrosslinked components intrinsic in adhesives and by failure of the adhesive components to stretch and, for solving such problems, not only the elongation percentages in the lengthwise and widthwise directions but also the elongation percentage in the direction of thickness is an important factor; and, they found that when the elongation percentage of the surface layer is put closer to that of the base layer by providing the interface between the surface layer and base layer with a proximity or gradient in elongation percentage, a proximity or gradient in molecular weight and/or a proximity or gradient in degree of crosslinking, either singly or in combination (including all) or by combining the base layer and surface layer or layers so that one or a plurality (including all) of such proximity or gradient features may be attained, it becomes possible to equilibrate the elongation percentages in the XYZ [lengthwise, widthwise and thickness] directions and thereby obtain the desired performance characteristics, without sacrificing the adhesive power.
It is an object of the present invention, which has been completed based on such findings as mentioned above, to provide an adhesive film which has good cuttability with respect to both the base layer and the surface layer or layers laminated thereon, hardly allows the formation of adhesive burrs in various shearing processes such as punching, perforating and slitting, is excellent in workability and is suited for use in continuous processing and, even in the case of a double-faced adhesive film with a thickness of 200 μm or less, for instance, can satisfactorily undergo such processing as the above-mentioned punching or perforating.
In the following explanation of the Summary, reference numerals are referred as of the Embodiment in order to easily read the present invention, however, these numerals are not intended to restrict the invention as of the Embodiment.